Time delay switches, which may delay either the change from the initial state to the complementary state or vice versa, are important in many electronic circuit applications. For example, this type of switch is useful in the telephone industry, because it permits a central office to disconnect subscriber equipment and test the lines between the central office and subscriber. Such switches provide a time delay between a control signal and the actual operation of the switch which, depending upon the type of switch, will be the change to or return from the complementary state. The latter is commonly called a slow release relay. The time delay assures a specified sequence of circuit operation and permits, for example, testing circuits for faults from a central location. To simplify the circuit operation, the switch should have constant, although different, impedances in both the initial and complementary states.
Several approaches have been taken in designing the slow release type of time delay switches. The use of electromechanical relays in such switches is well known. An article by R. L. Peck, Jr., Bell System Technical Journal, 33, 187-217 (1954), discusses relevant design considerations for a particular type of slow release relay.
Various types of time delay control are in common use. For example, bimetallic strips, air dashpots and solid state hybrids have been used in time delay relays. The latter approach is attractive because it affords possibilities of high reliability, small size and low cost because of its solid state timing function. However, it still requires a relay for the switching function. For many uses, the bulk and weight of this type of switch are drawbacks and alternatives which do not use electromechanical relays have been sought. Optical activation is a desirable feature because it does not compromise attainment of the desired characteristics while affording complete electrical isolation between circuits.